Generally, bicycle frames comprise a head pipe, a top tube, a down tube, a seat tube, a pair of back forks, and a pair of chain stay. Each back fork and each chain stay include respective fork ends at their rear portions, so that the rear hub is mounted across the fork ends. When a multistage speed change sprocket assembly is mounted on the rear hub, a derailleur for shifting a driving chain back and forth to desired ones of the sprockets is mounted on one fork end.
When the derailleur mounting portion at the fork end is not properly parallel with respect to the longitudinal center line of the bicycle frame, with which the rear hub is properly mounted, the positional relation between the chain guide at the derailleur and the sprocket is out of alignment, so that even when the derailleur operates properly, the speed change efficiency will deteriorate markedly.
Therefore, the parallel relationship between the longitudinal center line and the surface of the derailleur mounting portion should be strictly maintained. In order to check whether this parallel relationship exists, an adjuster is used which is provided with a threaded body screwable with a threaded bore at the derailleur mounting portion and an operating lever supported rotatably to the threaded body. The adjuster is screwably mounted on the derailleur mounting portion and a rim of a rear wheel is utilized to measure an interval between the side surface of the rim and the operating lever. When the measured degree of parallelism exceeds an allowable limit of error, a user grips the bicycle frame near the measured portion with one hand and the operating lever with his other hand and turns the lever toward or away from the rim, thereby adjusting the parallelism of the derailleur mounting portion with respect to the longitudinal center line of the bicycle frame.
The above-described adjuster for the fork end, however, cannot measure the aforesaid interval with accuracy because the rim is slanted at the side surface thereof with respect to the center line of the frame. As a result, the measured value of the interval is only a rough measurement, thereby creating a problem in that the fork end cannot be adjusted with high accuracy.